The present invention relates in general to a venous return cannula, and, more specifically, to a multi-stage cannula generating an even flow from multiple inlets along the length of the cannula.
A venous drainage cannula (also called a catheter) is used during cardiac surgery to remove (i.e., drain) blood from a patient using suction provided by gravity siphon or a pump coupled to the cannula. The removed blood is conducted to perfusion equipment which treats the blood by removing carbon dioxide, infusing oxygen, and heating or cooling the blood before returning it to the patient. It has become common to use a multi-stage cannula having separate sets of openings along the longitudinal body of the cannula to collect blood from locations near both the inferior vena cava and the superior vena cava, as is shown in U.S. Pat. No. 4,129,129 to Amrine, for example.
One reason why drainage from two locations is desirable for collecting blood during coronary artery bypass surgery is that blood flows into the right atrium in two directions (i.e., from the superior and inferior vena cava). The areas containing fenestrations (i.e., holes) are often referred to as baskets. Use of two or more separated baskets allow for a more dispersed intake of blood, helping to avoid pockets of blood that become stagnant and do not circulate through the perfusion circuit. The drainage flow volume passing through each basket depends upon the pressure drop at each stage of the cannula. Approximately equal amounts of blood are desired to be collected at each basket, so that the aggregate cross-sectional area of the fenestrations of each basket has been kept close to equal. Nevertheless, the volume of blood collected by the cannula from each respective basket has not been equal due to differences in pressure drop. Specifically, the volume collected at the first stage of a prior art cannula has been significantly less than at the second stage. The basket closest to the source of suction (e.g., the second stage in a two stage cannula) experiences a greater pressure drop and therefore provides the majority of the intake volume. Once it is inside the lumen of the cannula, blood passing through the fenestrations of the first stage is exposed to a bolus of blood that has already released pressure after passing through the second stage basket. The smaller pressure drop across the first stage results in a lowered intake so that the total volume of blood supplied by the cannula is unequally distributed between the stages.
Normal blood flow for a patient is typically characterized by substantially equal blood volumes flowing from the inferior and superior vena cava (e.g., in many adult studies, the inferior vena cava has been found to convey up to 65% of total cardiac output. Individual variations in age, size, and shape, or variations in specific surgical procedure that may be performed may result in slightly different flow ratios. An even flow as described herein includes, without limitation, flow ratios from 1-to-1 up to 2-to-1 or more.
The “flow coefficient” of a device is a measure of its ability to pass a fluid therethrough. It relates the fluid flow in volume per unit time to the inlet and outlet pressures and the pressure drop. Since prior art cannulae with multiple stages have had unequal pressure drops at the respective stages, manipulating the flow coefficients would not be successful in obtaining the desired flow ratios. When desiring to obtain even flow volumes from the regions of both baskets, it has been necessary to use two separate single-stage cannulae so that collection at one location does not affect the pressure drop at the other location. However, the use of dual cannulae requires a larger incision or multiple incisions. Therefore, it would be desirable to optimize the percentage of flow between multiple stages of a cannula, especially to allow control of a flow ratio to match blood collection to the characteristics of a particular patient and procedure so that blood is efficiently collected without creating stagnant pockets or collapsing anatomical structures onto any fenestrations.